Usually, the transmit chain of a radio communication network comprises a power amplifier to amplify a signal before transmission over the radio interface. The PAR is an important characteristic of the signal submitted to the power amplifier. The PAR is defined by the ratio between the highest amplitude of the signal and the average amplitude of the signal.
Wideband multi-carrier signals transmitted by one or several users using spread spectrum techniques (e.g. OCDM, OFDM) are presenting a near-Gaussian frequency distribution with a stringent spectrum mask and wide envelope modulation dynamic resulting in high Peak to Average Ratio (PAR about 12 dB).
A power amplifier is usually characterized by its efficiency defined as the ratio between the power of the signal at the amplifier output and the total power consumption of the power amplifier.
A signal having a high Peak to Average Ratio results in a low efficiency of the power amplifier. For a standard compliant amplification of a CDMA signal with a PAR about 12 dB, an over-dimensioning of the amplifier chain of the same order is required (known as power amplifier backoff of about 12 dB). Such an over-dimensioning causes big penalties in terms of required energy supply or appropriate cooling means at the transmitters of the radio communication network (especially at the base stations). This over-dimensioning is nevertheless necessary not to reach the non-linear domain of the power amplifier characteristics. If this happens, peaks in the signal would saturate the power amplifier causing high bit error rate at the receiver because of modulation distortion.
A known countermeasure to this problem is the clipping technique. It consists in eliminating from the signal the peaks having an amplitude above a threshold amplitude determined according to the characteristics of the power amplifier. This is usually obtained by saturating the signal at this amplitude threshold. The choice of the threshold value is a tradeoff between the power amplifier efficiency and the quality of the transmitted signal. Indeed, the lower the threshold is set, the more information contained in the peaks will be removed from the signal and the more the quality is decreased.
The usual clipping techniques have to face non linear filtering problems. On the one hand, saturating the signal at the threshold value generate wide band noise not compatible with requirements set to the signal in term of spectrum mask. Indeed, usual clipping techniques cause the signal spectrum to extend beyond the allowed spectrum mask causing interference in adjacent frequency channels. On the other hand, when filtering is performed to eliminate the noise outside the allowed spectrum mask, the memory effect intrinsic to filtering regenerates signal peaks above the desired threshold value.
Another known method consists in clipping the signal by subtracting a predefined clipping function from the signal when a given power threshold is exceeded. In order to ensure that clipping does not cause any out-of-band interference, a function having approximately the same bandwidth as the signal to clip is selected. This method gives good results as long as no peaks exceeding the threshold fall into the range of the clipping function of another peak.
FIG. 1a shows such a situation where the distance between the peaks exceeding the threshold is large enough not to cause interference between the clipping of different peaks. FIG. 1a illustrates the clipping functions δ1a(x), . . . , δ3a(x) for three peaks Peak1a, . . . , Peak 3a respectively separated by a distance Δ1a, . . . , Δ2a. The clipping functions δ1a(x), . . . , δ3a(x) are not overlapping since the distance between the peaks is large enough so that the intersection of the clipping function happens in domains where the clipping functions are very attenuated. Each of the clipping functions δ1a(x), . . . , δ3a(x) are multiplied with a scaling factor depending on the peak amplitude and calculated using a usual method.
FIG. 1b, on the contrary, shows a situation where a three peaks exceeding the threshold are concentrated in a small domain causing interference when the signal is clipped. FIG. 1b illustrates the clipping functions δ1b(x), . . . , δ3b(x) for three peaks Peak1b, . . . , Peak 3b respectively separated by a distance Δ1b, . . . , Δ2b. In this case, the clipping functions δ1b(x), . . . , δ3b(x) are overlapping. The power of the second peak Peak 2b is already decreased when clipping is applied either to Peak 1b or to Peak 3b. As a consequence, the usual scaling factor calculated for Peak 2b depending on its amplitude before any clipping is applied is too high. If such an effect is neglected peak 2b will be overclipped.
A solution to this inconvenient consists in performing several iterations of the clipping method, at each iteration only the highest peaks separated by a large enough distance (not causing overlapping of their clipping functions) are clipped in additional iterations the remaining other peaks are clipped. This method is described in European patent application EP 1 195 892.
An inconvenient of the previous described clipping method is that only a simplified interaction model is used to cope with overclipping causing an increased Peak Code Domain Error and an increased bit error rate.
The object of the present invention is to provide a method for scaling peak power amplitude in a signal that better deals with the reduction of overclipping.
Another object of the present invention is to provide a transmitter implementing this method.